Question

Cardiac muscles are immune to fatigue. Explain.​

Answer 1

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Cardiac muscle resists fatigue so well because it's got more mitochondria than skeletal muscle. With so many power plants at its disposal, the heart doesn't need to stop and chill out. It also has a steady supply of blood bringing it oxygen and nutrients.

Answer 2

the skeletal muscles in our legs and arms may be fatigued and possibly sore.

This is because of depletion of ATP in muscle required for muscle contraction. The primary ATP source for contraction is produced by mitochondria by aerobic respiration. Mitochondria make up, on average, about 1-2 percent of your skeletal muscle by volume, but this is generally enough to provide the needed energy for your daily movements. When depleted, ATP needs to be resynthesized from other sources, creatine phosphate and muscle glycogen (anaerobically) - Secondary source.

Our body use this Secondary source usually during either starvation or rigorous exercise. Let us concentrate only on glycogen anaerobic restoration.

Simple glycolysis were their is net gain of 2ATP,since their is absence of oxygen (during rigorous exercise their is improper supply of oxygen ) instead of converting pyruvate to acetyl coA, pyruvate is converted into lactate by the help of enzyme - lactate dehydrogenase/LDH (more or less the reaction is irreversible).

The lactate produced is an organic acid which causes acidosis of cytosol, decrease in pH(acidic) which in turn decreases oxygen and hemoglobin affinity - Bohr's effect. Muscles generate lactic acid so quickly that pH of the blood passing through the will drop to around 7.2, which causes haemoglobin to begin releasing roughly 10% more oxygen. And by this time creatine phosphate is also depleted. Due rigorous exercise their is also increase in temperature to maintain homeostasis body start's to sweat which in turn causes dehydration and iconic imbalance. - all this changes leads to muscle fatigue.

Therefore fatigue is caused due to lactate accumulation in muscle fiber. Lactic acid accumulation causes pain in muscle but not DOMS(delayed onset muscle soreness).

By now one would have concluded that cardiac muscle has more mitochondria and their is no lactic acid production in heart.

The conclusion is partially correct, cardiac muscle may contain up to 35 percent. This large volume of mitochondria supplies a steady source of energy right to your heart, and explains why your heart rarely needs to "rest" like your skeletal muscles do. And also heart is extensively vasculatured.

However cardiac muscle do produce lactate, as soon as it is produced lactate is converted back into pyruvate (reaction is reversible) and this is due presence of different lactate dehydrogenase (isoenzyme) which is more active compared to other LDH.

Discovery of this isoenzyme addressed the half century old question properly.

Note: Isoenzymes are that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters (e.g. different Km values), or different regulatory properties.

Note: the lactate produced in muscle (Since it is not a gluconeogeneic tissue) is transported to liver where LDH convert it back to glucose and transport back into muscle and stored as glycogen- cori cycle.